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MG TD TF 1500 - Supercharged spark advance

Good aftenoon, all:

Following hard on the heels of Stuart's question on spark advance, I'm wondering what the general consensus is on spark advance for a supercharged powerplant.

We're ready to start the little beast up in a week or so [finally!] and I seem to remember another thread indicating that 26 degrees of advance would be about right for the Judson at six pounds boost. We'll be running a pertronix, but otherwise the ignition remains unchanged. Head hasn't been shaved but it has been polished and ported, and we're running a 30 thou overbore, oversize valves on both sides, and a Crane 260/270 degree cam, so compression will be a little higher than stock, I think. There's a copper head gasket, so we're okay there.

We'll be running 91 octane. We can get 93 octane as an ehtanol blend, but I've always been a bit cautious about that.

Thanks for any info [again...]

Dave
Dave Jorgensen

Dave, I think the consensus on the the stock static setting is that it is a little retarded. That said I believe I would start out at the stock setting and carefully work my way up from there. The two biggest dangers when cramming the extra air are to much spark advance and a lean mixture.

LaVerne
LED DOWNEY

The head hasn't been shaved, but what does the head measure from gasket face to gasket face at the 4 corners?
Len Fanelli

I am not quite sure about Led's comment about a lean mixture if cramming more air into an engine would make the mixture weaker.

I have a Shorrock supercharger on my TD and, as it is between the carburettor and the intake manifold, the air which is being shoved into the motor is laden with petrol as well.

In my car's case, it doesn't matter a fig how much air the blower shoves in, because it is the correct petrol/air mixture as determined by the needle setting of the carb and is irrespective of the pressure or volume being supplied.
Geoff Love

Geoff:
Have you left your ignition advance as a stock setting? What are you running for fuel?
Dave
Dave Jorgensen

Yes, I left it just as it was when I fitted the blower. I use the highest octane fuel I can conveniently find at regular gas stations, and have been meaning to try to get some 100 oct. avgas, so far to no avail.

The car runs well. To give an example: a hill near here is about a 1/2 mile long with, (unusually for Pa.) a good smooth road surface. B.B. (before blower) I had to change into 3rd gear to make the top with about a 10 mph reduction from 60 to 50 mph. A.B. I can now maintain 60 all the way up in top gear and that with a higher rear axle of4.3:1. There is no hint of pinking.

However, there is one problem which may be linked to ignition but is more probably valves. Upon descending long hills, such as going down the afore mentioned incline, when my foot if off the accelerator ( an unusual occurance), there is a loud popping noise emenating from the exhaust pipe. In the dark, people following me have commented that there appears to be a fire in the exhaust system and flames are sometimes seen out the back. Apparently, it looks fairly dramatic.

When I was 16 and was allowed to own and drive a rudementary motorcycle,called then an autocycle, I owned a two stroke version. I discovered that by arranging a second Bowden cable to the needle in the Amal carb, I could lean out the engine and increase the speed substantially. Upon throttling back, the same effect as the TD could be observed. I took the effect one stage further by fitting a two way switch to the handlebars and tapping the exhaust's expansion box to take a sparking plug. This arrangement allowed me to cut out the plug in the cylinder and activate the one in the exhaust. The result was extremely satisfying, especially after dark when ragged blue/yellow flames, approximately 6 feet long could be induced. I had the truly wonderful experience of passing ourlocal constable who, in those days rode a pedal bicycle, at a speed well in excess of the then speed limit and giving him the benefit of my "jet blast". He fell off and my parents were informed.

By leaning out the mixture, the engine subsquently siezed solid and I then graduated to cars.

Hey ho...those were the days.
Geoff Love

Going down hill with the engine pulling in the fuel, I get the same pop... have a couple of favourite hills...!!!!
gblawson (gordon)- TD#27667

Geoff,

What you describe happened to me (Moss magnacharger, stock settings). Long hill, pop on the downside. Smell like cordite. I first thought it was the pop-off valve which is part of the Magnacharger setup. Eventually, it blew the manifold gasket right at the middle exhaust ports.

I put it down to ignition. When I went through everything upon changing the gasket, I found that the points were too wide. I'm still confused about how that happened. I had only about 600 miles since my last tuneup.

Seems fine now, but I don't feel confident and am still looking around for better answers.

Tom
Tom Balutis

Geoff, what I was getting at was that a "blown" engine is much more sensitive to mixture and ignition settings than a normally aspirated one. A lean mixture could melt a piston in a hurry. Compressing the mixture does not alter the fuel/air ratio but will magnify off the mark timming and carb settings.

LaVerne
LED DOWNEY

I have a Judson on my car and ran the timing as normal. Did not change anything on it. I do have a solid copper head gasket on the car as the regular head gasket will blow quite easily with the Supercharger running. Mine too blew right between 2 and 3. I had a custom made gasket to .060 thickness and it works fine. Also with the Judson, the way it oils using MMO I did not try and lean it out at all. Like everyone else to lean and pistons get holes in them and that aint good as they say.
TRM Maine

I recently had my distributor rebuilt and adjusted to work best with my Marshall-Nordec J-75. I was told that with a blower, the distributor advance curve should be limited at the higher RPMs to prevent piston burn. Also, the advised static timing is 10 to 13 degrees. Use of my Pertronix does make a slight difference in the curve so that went to him with the distributor. I have not had the car on the street yet so I cannot comment on its performance.
The website of the man who did the work is
www.advanceddistributors.com
He is very knowledgeable and does a fast turn-around.
Jim Merz

This thread was discussed between 09/02/2008 and 11/02/2008

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